1. Field of the Invention
This invention relates to the processing of semiconductor wafers. More particularly, this invention relates to improvements in the process of forming a selective coating of tungsten on a semiconductor wafer having a patterned mask thereon.
2. Description of the Related Art
In the processing of semiconductor wafers to selectively deposit tungsten over layers of, for example, aluminum, silicon, tungsten, or titanium nitride, having patterned masks thereon, it is important that the surfaces be free of any contamination, including moisture or any oxides such as, for example, aluminum oxide, silicon oxide, tungsten oxide, and titanium oxide.
Such contamination, if present can either mask or inhibit nucleation on the unmasked areas where the selective tungsten deposit is desired, or may provide unwanted nucleation sites on masked portions of the wafer.
The semiconductor wafer surfaces are, therefore, usually cleaned to remove all contamination and moisture prior to such a selective deposit. Such cleaning includes both the surfaces to be deposited on, as well as the mask surfaces.
Such cleaning is conventionally carried out by wet cleaning, for example, using an HF etch (for oxide removal) or a H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 :H.sub.2 O cleaning solution (for removal of organics) to remove the contaminants, including oxides, from both the unmasked surfaces as well as the surfaces of the mask.
While such wet cleaning of the masked and unmasked areas of the wafer surface does result in removal of the contaminants, including oxides, from the mask surfaces as well as the unmasked surfaces, moisture is not necessarily removed. More importantly, however, is the subsequent exposure of the cleaned surfaces to ambient conditions during the transfer of the cleaned wafer from the wet cleaning apparatus to a vacuum deposition apparatus suitable for the selective deposition of tungsten on the unmasked areas of the wafer by deposition techniques such as chemical vapor deposition (CVD) or plasma-assisted CVD. Such exposure can interfere with the subsequent tungsten deposition process, including the reproducibility of the results from wafer to wafer.
It would, therefore, be beneficial to provide an improved process for the selective deposition of tungsten on a semiconductor wafer wherein the masked and unmasked surfaces could be cleaned to remove contaminants such as oxides and moisture and the selective tungsten deposition then carried out without intervening exposure of the cleaned surfaces of the wafer to further contamination.